Fix and factor out the SNMPv3 security parameter descriptions

[lacatoire]

Fixes #3690

snmp3_real_walk() described privacy_protocol with the authentication protocols, which it does not accept. 1051736 had landed on the wrong varlistentry, overwriting privacy_protocol and leaving auth_protocol stale, and had skipped snmp3_set() and SNMP::setSecurity() entirely. All of it is repaired here, including the two missing 8.1.0 changelog entries.

These descriptions were duplicated by hand across six pages, which is how one commit desynchronised four of them unnoticed for three years. They are now defined once in language-snippets.ent, following the existing pgsql.parameter.* and pgsql.changelog.* patterns.

The wording is unchanged from the pages that were already correct, so snmp3_get(), snmp3_getnext() and snmp3_walk() render identically. Checked against a real SNMPv3 agent: DES, AES and AES128 are accepted for privacy_protocol, MD5, SHA, SHA256 and SHA512 for auth_protocol, anything else throws a ValueError.

Translations are unaffected: en/language-snippets.ent is layered as a fallback, so pages build whether or not they adopt the entities, rendering English until each translation defines its own, as was done for pgsql.parameter.*.